<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
          #father #son{
              color: blue;
          }
          #father p.c2{
              color: black;
          }
          div.c1 p.c2{
              color: red;
          }
          #father{
              color: green !important;
          }
    </style>
</head>

<body>
     <div id="father" class="c1">
        <p id="son" class="c2">
            试问这行文字是什么颜色的？
        </p>
     </div>
</body>

</html>